1. Field of the Invention
The present invention relates to the fabrication of axial, radial or spherical polymer gradient index (GRIN) lenses that possess either a fixed focal length or dynamically variable focal length. The lenses are supercomposite polymers formed by layering polymer composite films into hierarchical structures. They can be designed for use from the visible to the mm wavelength ranges.
2. Discussion of the Related Art
Gradient Index Optics are well known and are the subject of recent reviews. In a conventional lens, an incoming light ray is refracted when it enters the shaped lens surface because of the abrupt change of the refractive index from air to the homogeneous material. The surface shape of the lens determines the focusing and imaging properties of the lens.
In a gradient refractive index (GRIN) lens there is a continuous variation of the refractive index within the lens material. In a simple GRIN lens plane optical surfaces can be used. The light rays are continuously bent within the lens. The focusing properties are determined by the variation of refractive index within the lens material. There are two gradient index (GRIN) lens types described in the literature: axial gradient and radial/cylindrical gradient. In the axial gradient the refractive index varies in a continuous way along the optical axis of the inhomogeneous medium. In the axial gradient, the surfaces of constant index are planes perpendicular to the optical axis. In the radial/cylindrical the index profile varies continuously from the optical axis to the periphery along the transverse direction in such a way that the surfaces of constant index are concentric cylinders about the optical axis.
The simple geometry of a GRIN lens with flat surfaces allows the efficient production and simplified assembly of systems of lenses. Varying the thickness of the lens can vary the lens parameters such as the focal length and working distance. Thin lenses down to 0.02 mm in thickness are possible. Alternatively, the image plane can be made to lie directly on the exit surface of the lens.
A conventional lens with spherical surfaces and with a homogeneous index of refraction will not focus light perfectly; there will be spherical and chromatic aberrations. It is also well known in the art that these aberrations can be reduced or eliminated by employing axial gradient lens blanks. An axial gradient lens is a lens that has an index of refraction profile that varies in one direction only, usually chosen to be the optical axis. These aberration free lenses can be used advantageously in a variety of optical systems, such as slide projectors, cameras, binoculars, and many other imaging devices. The number of lens elements required for a given task can be reduced as well as the weight and complexity of the system.
U.S. Pat. No. 5,262,896, to R. Blankenbecler describes the fabrication of axial gradient lenses by the controlled diffusion process; the blanks for the fabrication of such gradient lenses can be made by a variety of processes such as SOL-GEL, infusion, and diffusion and may be glass, plastic or other suitable optical material. The above discussion applies to both radial and cylindrical lenses; however the grinding and polishing of cylindrical lenses to the needed precision is especially difficult.
U.S. Pat. No. 4,956,000, to Reeber et al describes a method and apparatus for fabricating a lens having a radially non-uniform but axially symmetrical distribution of lens material, in which the lens size and shape is determined by the selective direction and condensation of vaporized lens material onto a substrate.
U.S. Pat. No. 1,943,521, to W. Ewald describes a segmented lens built up of constituents of different indices of refraction. The separate parts of the lenses, each of which is homogeneous, are cemented together in such a manner that the boundary surfaces or interfaces are substantially located in the direction of the path of light rays. That is, the interfaces are parallel to the optical axis. The indices of refraction are chosen so as to reduce the spherical aberration of the lenses and produce clearly defined images on a screen.
U.S. Pat. No. 5,236,486, to Blankenbecler et al describes the forming of a cylindrical or spherical gradient lens blank from an axial gradient lens blank by heat molding (slumping). This process produces a monolithic lens with a continuous index of refraction profile.
A design for a cemented gradient index lens system for laser beam reshaping is disclosed by Wang et al in “Design of gradient-index lens systems for laser beam reshaping”, Applied Optics, 32, 4763–4769 (1993). A system using two axial gradient lenses and a homogeneous central transfer lens is disclosed. The interfaces between the front gradient lens and the central transfer lens and the central transfer lens and the rear gradient lens are spherical surfaces that must be ground and polished to fit into each other. In addition, the gradient index profiles are different and must be chosen properly to function as a beam reshaper.
Real time imaging sensors are critical for military tactical applications. Over the past few years, their use increased dramatically. A wide field of view (FOV) sensor is desirable for searching tasks, but for identification and tracking, a narrower FOV is required. A variable magnification telescope or zoom lens can provide a variable FOV. With current sensor systems, a change in FOV can be achieved, for example, by the insertion or removal of lens sets from the optical path. This is slow and requires bulky mechanical or electromechanical switching. The lens, in accordance with the present invention, gives continuous variation in the FOV with time constants of milliseconds or faster in a light weight, compact, lens.
Multilayer extrusion of polymers with several to thousands of layers is known. This extrusion process gives a material comprising many thousands of alternating layers of polymers, polymer composites, and/or polymers containing inorganic or metallic nanoparticles. The polymeric materials in the alternating layers can be chosen to have substantial differences in the index of refraction (Δn) so that the resulting materials will possess a modulation in the index with a period corresponding to the layer thickness. Layer thickness down to 5 nm can be readily produced. Nazarenko et al in “Polymer microlayer structures with anisotropic conductivity”, Journal of Materials Science, 34(7), 1461 (1999) and Mueller et al in Polymer Engineering and Science, 37(2), 355 (1997) describe the basic ideas for fabrication and the use of such materials to make dielectric reflectors. Methods of fabricating dielectric reflectors and filters with specific transmission properties and pass bands are described in P. Yeh; “Optical Waves in Layered Media”, Wiley, New York, (1998). Properly oriented layered birefringence polymers can give multilayer mirrors that maintain reflectivity over a broad band of incident angles.
A variety of methods have been developed for producing materials with a variation in the index of refraction that is suitable for GRIN optics. Polymer GRIN lenses are often fabricated by copolymerization (Y. Ohtsuka, et al, “Studies on the light-focusing plastic rod. 10:A light-focusing plastic fiber of methyl methacrylate-vinyl benzoate copolymer”, Applied Optics, 20, (15), 2726 (1981), and Y. Ohtsuka and Y. Koike, “Studies on the light-focusing plastic rod. 18: Control of refractive-index distribution of plastic radial gradient-index rod by photocopolymerization”, Applied Optics, 24(24), 4316 (1985)) of two different monomers undergoing diffusion. Incomplete diffusion leads to a composition gradient and hence an index gradient across the material. Most of these techniques result in small lenses, less than 10 mm diameter. The index gradients are small; the largest index variations are typically on the order of 0.01 to 0.03. Usually the index gradients are monotonic and the variation of index with distance is limited to those that can be achieved by the laws of diffusion. The largest radial polymer GRIN lens reported was made by this technique using a curved mold. It was 7 cm in diameter and had an Δn of 0.02 (Wu, S. P, Nihei, E., Koike, Y. “Large Radial Graded-Index Polymer” Appl. Opt. 35(1), 28 (1996)). Other techniques to produce a composition gradient include dopant diffusion and centrifugation. Complex mixing and extrusion techniques have also been proposed. The polymer copolymerization techniques are effective only if the components are miscible over all ranges of polymerization. This usually means the components are very similar polymers and the maximum Δn that can be achieved is small. Polymeric materials made by dopant diffusion are often short lived because of migration of the dopants. The mixing/extrusion techniques involve many control variables that are difficult to control and in addition they can only be used with polymers that are miscible over a wide range of compositions.
Generally, multilayer polymers have been fabricated using glassy polymers. Elastomeric multilayer structures with layer spacings suitable for dielectric filters and reflectors have been fabricated by sequential spin coating and by multilayer extrusion.
In addition to the military applications, the lenses of the present invention will have widespread commercial applications where light weight lenses with short long and variable focal lengths are required; for example, zoom lenses for reading glasses cameras and binoculars.
In accordance with the present invention, the index gradients (Δn) can be specified independently. This allows flexibility in the focal properties of the lens that was not previously possible in a single lens. It also allows a lens to be designed with aberration corrections. A simpler improved and more flexible method to fabricate axial, radial or spherical polymer gradient lenses exhibiting the above properties is highly desirable.